Osama Abu Zayd, a legal adviser for an umbrella group of rebel factions known as the Free Syrian Army, said the cease-fire went into effect Tuesday evening.

He said the first batch will begin evacuating later Tuesday.

Yasser al-Youssef, a spokesman for the Nour el-Din el-Zinki rebel group, confirmed the cease-fire and saidd the goal is to evacuate civilians and rebels from besieged areas.

There was no immediate comment from the Syrian government or Russia on the reports.

The decision came after U.N.'s children's agency warned that dozens of unaccompanied children were trapped in a building under fire in eastern Aleppo and called for their immediate evacuation from the rebel enclave.

UNICEF said in a statement on Tuesday that there could be more than 100 children trapped in the building.

UNICEF regional director, Geert Cappelaere, said it's "time for the world to stand up for the children of Aleppo and bring their living nightmare to an end."

Pro-government forces have launched a ferocious assault on Aleppo's few remaining opposition-held neighborhoods, trapping thousands of civilians under unrelenting heavy fire. There are unconfirmed reports that government forces are killing civilians.

Cappelaere said that UNICEF is "deeply concerned" over the unverified reports of the "extrajudicial killings of civilians, including children."

The U.N. human rights office said it received reports of Syria's pro-government forces killing at least 82 civilians as they entered the last remaining strongholds of the rebels in eastern Aleppo.

Spokesman Rupert Colville of the U.N. human rights office said the reports recount pro-government forces entering homes and killing some civilians "on the spot" in the former rebel enclave.

Colville spoke to reporters in Geneva on Tuesday.

He said 11 women and 13 children were among those reportedly killed in four neighborhoods of the increasingly-shrinking rebel enclave in the city of Aleppo.

Colville said the reports came in late the previous evening and that he doesn't know exactly when the killings took place.